Again, there's some stupidly obvious choices here: SIN CITY, BATMAN: YEAR ONE, DAREDEVIL (though much of it is OOP), DARK KNIGHT RETURNS, but I'm going to go with a slightly less obvious one here:

HARDBOILED

Created with Geof Darrow in 1992, HARDBOILED is a masterpiece of over-the-top detail and carnage. Really the star here is Darrow, with more detail-per-square-inch than any five other comics combined. There are pages here you can stare at for five minutes each, pulling out details.

There's a level of outrageous here that I'd never seen before in a comic before this -- where crazy background details do more for world-building than anything previous. And there's a crazy level of kinetic cartoon violence going on.

I wouldn't say the story is particularly DEEP, but any comic that works out as ROBOCOP-meets-WHERE'S-WALDO is AOK in my book!